1. Field of the Invention
The present invention relates to a process for polymerizing .alpha.-methylstyrene, acrylonitrile, styrene, and copolymerizable vinyl monomer in the presence of diene type synthetic rubber.
2. Description of the Prior Art
Heretofore, there have been proposed several methods for polymerizing monomers composed mainly of .alpha.-methylstyrene, acrylonitrile, and styrene in the presence of diene type synthetic rubber. These conventional methods, however, suffer from disadvantages that the resulting copolymer is not sufficient in impact resistance when it contains .alpha.-methylstyrene in a large amount, and is not sufficient in heat distortion resistance when it contains .alpha.-methylstyrene in a small amount. In order to overcome these disadvantages, there was proposed a new process in Japanese Patent Publication No. 14936/1969. However, this process is still insufficient in heat distortion resistance when the content of .alpha.-methylstyrene is less than 50 parts by weight.
In order to overcome the above-mentioned disadvantages, the present inventors carried out a series of researches which led to the finding that it is possible to obtain a thermoplastic resin which has high impact resistance and heat distortion resistance and low discoloration by heat, in such way that a monomer mixture composed mainly of acrylonitrile and styrene is subjected to polymerization in the presence of diene type synthetic resin, and subsequently a monomer mixture composed mainly of .alpha.-methylstyrene is added to the polymerization system, and finally a monomer mixture composed mainly of acrylonitrile is added little by little so that the polymerization is substantially completed.